Just for the record: ANSI X3.171-1989 here next to me says that the
outer dimensions of the familiar cartridge are 90.0 x 94.0 x 3.3 mm and
that the magnetic disk material itself has a diameter of 85.80 mm.
Indeed, nowhere in the drawings appears a measure of 3.5 inches.

Peter Westlake <peter@harlequin.co.uk> wrote:
>Why are fractions only usable with Imperial units? Do ISO standards
>forbid metric units to be written as fractions?

ISO 31-0 strongly suggests that SI units should be used only with
decimal fractions, because this significantly increases user convenience
and safety. It is obviously much easier for humans to compare 0.2734375
and 0.28125 than 35/128 and 9/32. Use of fractions in technical
drawings, product documentation, etc. is today widely considered to be a
gross violation of good engineering and documentation practice. Infinite
decimal fractions such as 1/3 are trivially avoided in real life by
making design dimensions multiples of a module size (such as 300 mm in
building construction) with a sufficient number of prime factors, as any
well-trained technical designer will be happy to tell you. Vulgar
fractions are as antiquated, inappropriate and useless in the 21st
century as are the inch-pound Flintstone units. It is amazing, how one
locale manages to need over a century longer to grasp this than all
others.

Good official style guides for using units of measurement are available
on: